L-Amino acids, in particular L-threonine, are used in human medicine and in the pharmaceutical industry, in the foodstuff industry and, very particularly, in animal nutrition.
It is known that L-amino acids can be prepared by fermenting Enterobacteriaceae strains, in particular Escherichia coli (E. coli) and Serratia marcescens. Because of the great importance, efforts are continually being made to improve the preparation methods. Methodological improvements can include measures relating to fermentation technology, such as stirring or supplying with oxygen, or the composition of the nutrient media, such as the sugar concentration during the fermentation, or the working-up to the product form, for example by means of ion exchange chromatography, or the intrinsic performance properties of the microorganism itself.
Methods of mutagenesis, selection and mutant choice are used for improving the performance properties of these microorganisms. This thereby results in strains which are resistant to antimetabolites, such as the threonine analog α-amino-β-hydroxyvaleric acid (AHV), or auxotrophic for metabolites of regulatory importance and produce L-amino acids such as L-threonine.
For a number of years now, recombinant DNA methods have also been used for improving L-amino acid-producing strains of the Enterobacteriaceae family by amplifying individual amino acid biosynthesis genes and investigating the effect on production. Compiled information on the cell biology and molecular biology of Escherichia coli and Salmonella can be found in Neidhardt (ed): Escherichia coli and Salmonella, Cellular and Molecular Biology, 2nd edition, ASM Press, Washington, D.C., USA, (1996).